Descriptive Epidemiological Study of Koala Retrovirus at the San Diego Zoo Over a 20-Year Period
Koala retrovirus (KoRV) is an important cause of mortality in wild populations and zoo-based koalas in Australia, Japan, Europe, and North America, with deaths resulting from neoplasia and disease from opportunistic pathogens.1 The goal of this study was to describe the basic epidemiology of KoRV among 118 koalas housed at San Diego Zoo (SDZ) from 1993–2012. Necropsy reports were used to identify cases of KoRV based on the following: lymphoid neoplasia (n=9), aplastic anemia (n=7), opportunistic infections (e.g., bronchopneumonia, cryptococcal inflammation, pseudomonas infection; n=3), osteochondromatosis (n=1), and other diseases (hyperthermia, nasal cavity osteoma, severe proliferative vasculopathy, arthropathy n=4). Incidence and proportionate mortality were estimated. Among koalas that died, exact logistic regression was used to estimate the unadjusted odds ratios to determine the association between identified demographic characteristics and KoRV-related mortality. The incidence rate was 8.8 cases per every 100 koala-years at risk (51 cases/581.6 koala-years at risk). Proportionate mortality for KoRV was 47% (24 cases/51 mortalities). Deaths in older koalas were less likely due to KoRV than deaths in younger koalas; the odds of KoRV-related mortality decreased by 12% for every increasing year in age (OR=0.88; 95% CI: 0.78–0.98; p=0.02). Longer time periods in the SDZ cohort were also protective from KoRV-related death (OR=0.90; 95% CI: 0.80–1.0; p=0.04). No significant associations (p>0.05) were identified between KoRV mortality and sex, birth location (SDZ vs. born at another institution), and birth year. Adjusted analyses are underway to identify concurrent factors associated with KoRV-related deaths.
1. Denner J, Young P. Koala retroviruses: characterization and impact on the life of koalas. Retrovirology. 2013;10:108.